The E2A gene products belong to a class of helix-loop-helix (HLH) proteins, also named E-proteins. Members of the E-proteins include E12, E47, E2-2 and HEB. The DNA binding activities of the E- proteins are regulated by a distinct class of antagonistic HLH proteins, named Id gene products. The E2A proteins act at multiple stages during B cell development. In the lymphoid-primed multipotent progenitor and common lymphoid compartments, the E2A gene products act together with PU.1, EBF and Pax5 to specify a B cell fate. They maintain the expression of EBF and Pax5 in pro-B cells. In pre-B cells, the E2A proteins activate Ig: VJ gene rearrangement. In the immature-B cell compartment the E2A proteins induce receptor editing. E2A protein levels decrease upon innocuous BCR expression whereas E2A levels remain high in response to self-antigen. E2A abundance at this stage is regulated through a post-transcriptional mechanism that involves differences in protein stability. At the mature-B cell stage E2A levels are low but are elevated in response to BCR or TLR- mediated signaling, to promote class switch recombination (CSR). Here we propose to further analyze the activities of the E2A proteins at the immature- and mature-B cell stage. We would measure E12 and E47 turn-over rates in immature-B cells, expressing either an innocuous or self-reactive receptor. We would determine the individual roles of E12 and E47 at the immature-B cell stage. We would examine how the E2A proteins modulate receptor revision by identifying E2A targets in B cells expressing either an innocuous or self-reactive receptor. We would complement this analysis by performing a genome-wide screen for E2A binding sites. We would determine how the E2A proteins act with other families of transcriptional regulators to modulate down-stream target gene expression using a bioinformatics approach. We would examine the regulation and role of E-proteins in CSR. We would identify E2A binding sites, using a genome-wide screen, in naive and activated B cells. Our ultimate goal would be to identify the cis- regulatory codes that underpin the response to either tonic or auto-reactive or non-self reactive BCR- mediated signaling.